The brain-Box 1

	Published by Australian Academy of Science
Getting our heads around the brain Box 1 \| The human nervous system

The nervous system is made up of specialised cells that communicate with each other and with other cells in the body. These specialised cells are called nerve cells or neurons.

The nerves of the body are organised into systems. The central nervous system consists of the brain and spinal cord. The peripheral nervous system is a vast network of nerves that extend to all parts of the body, linking with the spinal cord through 31 pairs of spinal nerves. The two systems function together, with nerves from the periphery entering and becoming part of the central nervous system, and vice versa.

There are three kinds of neuron in the peripheral nervous system: sensory, motor and autonomic. Sensory neurons are responsible for bringing information about changes inside and outside the body to the central nervous system. Sometimes the spinal cord can make decisions without any need to consult with the brain – the 'knee-jerk' reaction caused by a doctor tapping the tendon that connects the kneecap to the shin bone is a classic example of this. More complex information needs to be interpreted by the brain, which then issues instructions via motor neurons to skeletal muscles for appropriate action.

Internal organs, such as the heart, lungs, gut and glands, are not under conscious control. The neurons that serve these organs form the autonomic, or involuntary, nervous system. This system is a part of the peripheral nervous system.

Structure of neurons

Neurons vary so much in shape that it isn't possible to describe a 'typical' one, but they do have three major features in common. Each has a cell body containing a nucleus and an extension, the axon, which transmits nerve impulses to other cells. The third major feature of neurons are one or more (usually numerous) fine, branching extensions called dendrites. They receive nerve impulses from other cells.

Neurons are connected to other cells

If you step on something sharp, you normally withdraw your foot - and straighten your other leg to maintain your balance. Neurons in the affected foot must therefore be connected to neurons in both legs.

In very simple connections in the nervous system, a single string of neurons is arranged end to end, with the axon of one ending on a dendrite of the next. Usually the connections are more complicated than this: a single neuron may have as many as 20,000 connections to other neurons. Not all of the connections in the nervous system are between two neurons – they can also connect to muscles or glands.

Transmission of a nerve impulse

When activated, neurons transmit a wave of electrochemical change. This wave of change is called an impulse. The starting point of an impulse could be a sense organ such as the skin, an eye, an ear, the nose or the tongue, or it could be at a dendrite that has received a message from another neuron. When a neuron is stimulated, it transmits the impulse electrically along its axon. At the end of the axon the impulse travels across a tiny gap, called a synapse, to another neuron (or to a gland or a muscle) by means of special chemical messengers called neurotransmitters.

The neurotransmitters affect the next cell in one of two ways: they either 'excite' it, so that it will send the impulse to the next cell, or they 'inhibit' it. The neurotransmitter molecules either break down or are reabsorbed after they have delivered their 'message'.

The electrical transmission of a nerve impulse is basically the same in all instances. But at the junction between cells, the chemical transmission of the impulse provides the capacity for differentiating between messages.

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